The present invention relates in general to bellows types of differential pressure measuring devices such as employed in connection with fluid-flow meters and recorders, and more particularly to new and useful improvements thereto for increased ease of manufacture and field maintenance of the devices as well as increased reliability in the operation thereof.
As is well known in the art, differential pressures derived from orifices, venturi tubes and like apparatus, may be employed to determine the rate of flow of fluids through conduits. Also, differential pressures may be employed to determine the level of fluids within closed tanks, as well as pressure drop or pressure increase across fluid-handling apparatus such as filters, scrubbers, pumps or compressors. Of the many known types of apparatus for differential pressure measurement, or for actuation of measurement devices in response to differential pressures, such as for example, mercurial or water manometers and elastic metallic membranes or diaphrams, those based on the principal of the deflection of a single, elastic metal bellows across which the differential pressures to be sensed are directly applied have been found to offer the greatest advantages in structural simplicity and operational sensitivity and accuracy. To provide a permanent record, the measured pressure differential is converted to a readout on a recording chart through suitable linkage mechanisms.
One environment in which apparatus for differential pressure measurements is used is in the natural gas industry where the flow of natural gas from the natural gas wells and fields needs to be accurately and dependably measured and recorded, especially in view of the economic considerations involved because of the increased cost of natural gas. For measurement of the gas flow through any particular pipe or pipeline, a differential pressure-responsive device or meter is connected across a primary device (venturi, orifice plate or flow tube) operatively located in or with respect to the pipeline. For recording or indicating functions, the mechanical output of the differential pressure-responsive device is connected to a recorder, indicator or other readout instrument. For controlling functions, the output of the differential pressure-responsive device is connected to a switch, transmitter or other control device to control the flow of gas through the pipeline.
U.S. Pat. No. 3,422,680, commonly assigned, which issued on Jan. 21, 1969 and which is herein incorporated by reference, discloses a single, elastic metal bellows type of differential pressure-responsive device. Although the device disclosed in U.S. Pat. No. 3,422,680 operates satisfactorily, there are areas open for improvement relative to ease of field maintenance and manufacture and assembly together with improved reliability.
Gas companies are concerned about obtaining an accurate and dependable reading of the amount of gas flowing so the recipient of the gas may be properly billed. Normally the gas continues to flow even though the differential pressure-responsive device may be providing an incorrect indication. Gas companies normally require that the differential pressure-responsive devices or meters be checked and inspected in accordance with a predetermined schedule. For mounting purposes, the prior art differential pressure-responsive devices are usually configured with a downwardly facing cylindrical socket (located on the lower portion of the unit) for a telescopic coupling connection to the upper end of a suitable meter supporting columnar member. The differential pressure-responsive device or meter is connected across the primary device (venturi, orfice plate or flow tube) by a high pressure connection pipe and a low pressure connection pipe. In order to check and inspect the internal parts or elements of the differential pressure-responsive device for dirt, trash and malfunctioning parts and also to clean the internal portions and further to replace internal parts, the prior art devices must be disconnected from the high pressure pipe, the low pressure pipe and the supporting column member and then be essentially completely disassembled in order to obtain access to the internal cavities and parts or elements of the device. Then, of course, the place the device back in service and on the line, the reverse of the above-listed procedure must be performed. While the device is off the line and out of service, the gas company is providing gas without being compensated for same and this is a very important consideration because of the dollar value of the gas. For example, if a well was providing 300,000 cubic feet of gas per day and gas was $5 per 1000 cubic feet and the meter was down for 2 hours then the gas company has lost $125 from just the one well or line. Because of the large quantity of fairly small parts of items which must be handled in the disassembly and assembly of the device, the overall operation is time consuming and therefore expensive. Since the devices are usually located at gas well sites in remote areas and subject to extreme weather conditions, the disassembly and assembly efforts may be extremely difficult, especially when protective clothing needs to be worn.
Also, a ball-disc drive is located in a lower cavity of the prior art device and drives an output shaft. The output shaft is connected to an external recorder, indicator or other readout instrument. If the output shaft is bent during the installation of the external recorder, indicator or other readout device then, of course, the bent shaft must be replaced. In order to replace the bent shaft, the prior art differential pressure-responsive device or meter must be removed from the lines and mounting post and then be disassembled in order to gain the necessary access to the damaged shaft.
A low-pressure relief valve is provided in the prior art device as an integral part of the main pressure body and is located where the valve is exposed to any liquids and dirt which might enter the meter system. The prior art relief valve comprises a number of small parts including a spring with a low value of spring force which allows the valve to be activated by less then 3.66 pounds of differential pressure which results in a tendency for the valve to leak if the pressure on the high side of the valve is no greater than 3.66 psi. Also, dirt and trash in the prior art device could easily come to rest between the valve ball and the valve seat, thereby causing the valve to leak and cause the meter to give an incorrect reading.